1. Field
Embodiments discussed herein relate to an image forming apparatus and, in particular, to an image forming apparatus including a recording head for ejecting liquid droplets.
2. Discussion of the Background
Image forming apparatuses are used as printers, facsimile machines, copiers, plotters, or multi-functional devices having two or more of the foregoing capabilities. As one type of image forming apparatus employing a liquid-ejection recording method, for example, an inkjet recording apparatus that uses a recording head (liquid-droplet ejection head) for ejecting liquid droplets is known.
In such a liquid-ejection-type of image forming apparatus, when liquid is not ejected from nozzles of the recording head for a long time, micro-vibration pulses (non-ejection pulses) may be applied to the head to vibrate the meniscus of liquid in the nozzles, without ejecting liquid droplets from the recording head so as to maintain the condition of the nozzles.
To maintain the condition of the nozzles of the recording head, such an image forming apparatus may also perform, during printing, a dummy ejection operation (also referred to as a flushing operation) in which liquid droplets not contributing to image formation (dummy ejection droplets) are ejected from the nozzles of the recording head.
In addition, such an image forming apparatus may be a line-type image forming apparatus capable of printing images on a continuous recording medium, such as a rolled sheet of paper, a continuous sheet of paper, a continuous-form paper, or a web medium. In such a case, unlike a case where cut sheets are used as the recording media, the dummy ejection operation cannot be performed between conveyed recording media. Hence, the image forming apparatus performs one of a first-type dummy ejection operation (referred to as a line flushing operation) to perform dummy ejection on a non-image-forming area per a constant length of the continuous recording medium, or a second-type dummy ejection operation (referred to as a star flushing operation) to eject a less-visible size of liquid droplets for dummy ejection on an image-forming area of the recording medium.
Moreover, a common driving waveform including a micro-driving signal, which vibrates the meniscus of liquid slightly without ejecting droplets, and an overflow driving signal, in which the ink in a nozzle overflows the circumference of the nozzle, but does not become a droplet, is known. In addition, by applying the overflow driving signal, a foreign particle or ink mist can be drawn into the nozzle.
Note that the dummy ejection operation mentioned above discharges the ink with viscosity increased, is not used for printing, and has a role that maintains the nozzle so that normal ejection can be always performed.
However, in the case of using a continuous recording medium, such as rolled paper, the continuation ejection time may be several hours in many cases, and a nozzle omission (non-ejection) may occur under the influence of minute foreign substances, such as minute ink droplets (ink mist), paper powder, and fibers, which adhered near the nozzle during printing. When a droplet ejection operation is continued with the occurrence of the nozzle omission, a lump of ink mist may become large gradually and may fall on the recording medium.
It is difficult to prevent such deposition of ink mist near the nozzle completely in the usual dummy ejection operation.
In this case, using a common driving waveform containing the overflow driving signal, the overflow driving signal may be applied during printing (image formation). However, in this case, the length of the common driving waveform becomes long, driving frequency is lowered, and the print speed on a continuous recording medium is decreased.